Catalysts used industrially for gas-phase catalytic oxidation reactions are generally used for a certain period of time in relation to regular maintenance scheduling of the plant facility or the like. After the time of use has expired, the catalyst is removed from the reaction tube and is replaced with a newly produced catalyst. Catalyst poisoning by impurities contained in the reaction gas and formation of carbides are observed in the removed catalyst and on the reaction tube. Such substances cause a decrease in reaction yields and an increase in the rate of pressure loss during the time of use of the catalyst. Considering such problems, various solutions such as below have been proposed.
Patent Document 1 describes a technology where a treatment agent is arranged above the catalyst layer in a reaction tube or on the upper portion of an upper tube sheet in the reactor so that inhibitors produced in previous reactions are blocked from attaching to the catalyst for subsequent reactions. Patent Document 2 describes a technology where an inert substance layer is arranged by packing an inert substance between the first catalyst layer and the subsequent catalyst layer so that when a reaction gas from previous reactions passes through the inert layer, impurities contained in the reaction gas are removed to prevent deterioration of catalytic activity in the subsequent catalyst layer. Patent Document 3 describes a technology where an inert support coated with a molybdenum compound is used. Such a technology is effective, but cannot yield sufficient reaction results, thus causing an increase in production costs. Accordingly, further improvement is necessary. Patent Document 4 describes using a mixture of a catalyst and an inert substance so that hotspot phenomena are prevented during oxidation reactions and the selectivity of the catalyst and its life are improved. In addition, Patent Document 5 describes a technology where an inert substance or a packing material for a preheating layer is packed near the inlet to the catalyst layer so that the reaction gas is cooled or preheated, and an inert substance is packed near the outlet of the catalyst layer so that the residence time of the gas in an empty space is shortened and consecutive reactions are suppressed.